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Chief Superintendent Andy Brittain said that police vehicles, paramedics and an air ambulance were dispatched, but the girl was pronounced dead at the scene shortly after.

Around 75 minutes later, Brittain said that a 17-year-old male suspect who may have known the victim was arrested near Croydon.

Brittain said the suspect, who wasn't identified, was in custody and added that police weren't searching for anyone else in connection with the killing based on the information they had.

“This is every parent’s worst nightmare, and I know the officers who responded this morning, along with our emergency service colleagues, are devastated at the victim’s death," he said near the scene of the stabbing.

“This is an emotion I share and I know people across Croydon will be feeling the same," he said.

A white forensic tent was erected within a police cordon at the scene.

Stabbings involving young people aren't uncommon in London, but it is relatively rare for such incidents to involve girls.

London Mayor Sadiq Khan said that he was “heartbroken� by the death.

The Met's top police officer Mark Rowley, who spent about an hour at the scene, said the “senseless murder of a 15-year-old girl on her way to school is impossible to comprehend."

The girl, who hasn't been named, was a student at the Old Palace of John Whitgift School, a private girls’ school in Croydon.

The school expressed its shock at the “senseless and tragic death of our much-loved and valued friend and pupil.�